The 1581 supports a poor imitation of directories which are
really just partitions and largely unused. It also supports
burst loading like the Commodore 1571, but is actually
faster as it is better designed. It has 3160 blocks free
when formatted.

The 1581 is the highest density C64 serial bus drive made by
Commodore. However Creative Micro Designs (CMD) make the
FD2000 (1.6MB) and (until recently) the FD4000 (3.2MB)
3.5" disk drives. GEOS users like 1581s as they are very
fast when used with GEOS.